1. Field of the Invention
The present invention relates to a valve drive mechanism and, more particularly, to a valve drive mechanism configured to control the open period of an intake valve or exhaust valve.
2. Description of the Related Art
Valve drive mechanisms for engines that are capable of continuously controlling the open period and lift amount of an intake or exhaust valve have been used in many practical applications. An example of such a valve drive mechanism is disclosed in German Patent No. DE 4223172 C1. This mechanism includes a rocker cam that is supported for rocking motion and is driven by a camshaft. An intake or exhaust valve is opened and closed by the rocker cam. To continuously change the open period and lift amount of the valve, the center of the rocking movement of the rocker cam is varied.
In the prior art, there is a problem in that, at the time of starting an engine by cranking, or immediately after the engine starts, unburned fuel remains in engine. This unburned fuel can be undesirably discharged or can cause unstable combustion. This, in turn, can result in an undesirable large amount of discharge of, in particular, hydrocarbons (HC).